1. Field of the Invention
The manufacture of hard cheese is a long and tedious process, beginning with the inoculation with starter culture into raw or pasteurized standardized milk to the final stages of salting, packaging, and aging. Coagulation of milk and curd development are critical and time consuming steps in cheese manufacture. This invention relates to accelerating the manufacture of hard cheese by subjecting milk inoculated with rennet and starter culture microorganisms to high pressure carbon dioxide.
2. Description of the Prior Art
Over several millennia of cheesemaking, advances in the art have been incremental and relatively few. Traditionally, cheese was made from naturally-soured milk. Rennet obtained from calf stomach vells was used to generate the milk curd which was later separated from the whey. Beginning in the late nineteenth century, the concept of replacing soured milk with pure microbial lactic cultures was recognized as a means of better controlling the cheese characteristics, particularly the flavor and texture. Other improvements in cheese making included pasteurization of the milk and standardization of rennet by refinement of its extraction from calf vells. The latter modification resulted in a more uniform quality of curd and reduced curd contamination by micro-organisms associated with the vells.
In the modern process for making hard cheese, standardized pasteurized "cheese" milk, a starter culture, and rennet are combined and then heated to produce a coagulum. This coagulum is cut and heated further to separate the curd from the whey. The curd is then recovered and is subsequently washed, salted, packaged, and appropriately aged.
Rennet, containing the enzyme chymosin and/or pepsin and obtained from animal, plant, or microbial sources, is recognized as having multifunctional roles in the cheese-making process; it is responsible for proteolysis. Initially, rennet destabilizes the casein micelles by cleaving a specific site in the protein chain of k-casein; this is followed by coagulation in about 30 minutes. During aging, rennet and other enzymes from starter cultures continue to proteolize the proteins to peptides that impart characteristic flavor to cheeses. Proteolysis also releases fat globules from the curd matrix, making them available to the action of lipases.
Lactic acid starter cultures play important and complex roles in cheesemaking. Of course, their principal function is the production of lactic acids that precipitate the casein. In some modern processes, precipitation is accelerated by the addition of acids, such as hydrochloric or sulfuric acid. Casein precipitates at the isoelectric point of pH 4.6 and approximately 20.degree. C., conditions at which the negative charges on the surface of the casein micelles are neutralized. Other environmental factors such as ionic strength, dielectric properties of the solvent, and temperature also affect the solubility of proteins and can be used to advantage for protein precipitation.
Jordan et al. [N.Z.J. Dairy Sci. Technol., 22:247-256, (1987)] have shown that it is possible to precipitate casein by dissolution of carbon dioxide in milk. The reversible reaction for the dissolution of carbon dioxide in water or milk is expressed as follows: EQU CO.sub.2 +H.sub.2 O.revreaction.HCO.sub.3.sup.- +H.sup.+
Increasing the pressure of carbon dioxide injected into the milk results in an increased production of H.sup.+ thereby lowering the pH and causing coagulation of the protein.
Tomasula et al. [J. Dairy Sci., 78:506-514, (1995)] reported that when CO.sub.2 was sparged through the milk (maintained at 38.degree. C.) to a pressure of 5.52 MPa and held for 5 min, the separated whey had a pH of 6.0 and the resulting casein mass was granular, moist, friable, and contained higher levels of solids, ash, and calcium. Indications from carbon milk studies suggest that this treatment would have a strong antimicrobial effect on psychrotrophic bacteria in the milk [Amigo et al., Z. Lebensm. Unters. Forsch., 200:293-296 (1995); King and Mabbit, J. Dairy Res., 49:439-447 (1982); Montilla et al., Z. Lebensm. Unters. Forsch., 200:289-292, (1995); Raus-Madicdo et al., J. Food Prot., 59:502-508, (1996); Roberts and Torrey, J. Dairy Sci., 71:52-60, (1998)]. Whether or not the CO.sub.2 processing environment would be detrimental to the facultative anaerobic lactic acid bacteria commonly used in cheesemaking or to the multifunctional roles of rennet is less certain.
High hydrostatic pressure is known to have severe effects on microorganisms [Hoover et al., Food Technology, March 1989, pp. 99-107]; cell morphology was affected at pressures as low as 0.6 MPa, hydrophobic interactions were disrupted at pressures below 100 MPa, and biochemical reactions and membrane integrity were altered at pressures above 100 MPa. Compared to salt buffer or meat, milk provided some protection to microorganisms when exposed to high pressure [Styles et al., J. Food Sci 56:1404-1407 (1991); Patterson et al., J. Food Protection 58:524-529 (1995); Gervila et al., J. Food Protection 60:33-37 (1997)]. When inoculated into milk, Vibrio parahaemolyticus was inactivated at pressures over 165 MPa [Styles et al. (1991)], Listeria monocytogenes was killed at pressures over 300 MPa [Styles et al. (1991)], and Listeria innocua was reduced 7 to 8 log units at pressures over 450 MPa [Gervila et al. (1997)].
The effects of high pressure on milk proteins and enzymes essential to cheesemaking has been more varied. Beta-lactoglobulin was denatured at pressures over 100 MPa [Lopezfandino et al., J. Dairy Sci. 79:929-936, (1996)] while rennet and caseins were not denatured at pressures up to 130 MPa [Ohimiya et al., J. Food Sci., 52:84-87, (1987)] and the activity of plasmin or the structures of alpha-lactalbumin or bovine serum albumin were not altered at pressures up to 4,000 MPa [Ohimiya et al., Agric. Biol. Chem., 53:1-7. (1989)]. Pressures between 130 and 300 MPa delayed the aggregation of casein micelles [Ohimiya et al., (1987)], shortened the time for curd formation [Lopezfandino et al. (1996); Ohimiya et al., (1987)], and altered rennet coagulation of milk [Desobry-Banon et al., J. Dairy Sci., 77:3267-3274 (1994); Ohimiya et al. (1989)]. Pressures above 300 MPa increased cheese yields [Lopezfandino et al. (1996)].
In view of the potential risks to the functionality of rennet and starter cultures, steps in the processing of hard cheese that would subject these components to unusual conditions normally would not be contemplated.